NEW YORK, Sept. 30 – U.S. Secretary of State Mike Pompeo was on the July phone call where President Donald Trump asked his Ukrainian counterpart to investigate Joe Biden and his son, a senior State Department official told NBC News.

The July 25 phone call between Trump and Ukrainian president Volodymyr Zelenskiy and a related whistleblower complaint are now at the center of House Democrats' impeachment inquiry.

At the call, Trump told Zelenskiy that Biden's conduct sounded "horrible" to him. It's not unusual for the nation's top diplomat to be on a president's call with a foreign leader, but Pompeo has not acknowledged his involvement.

U.S. special envoy to Ukraine Kurt Volker resigned on Friday, 1 day after the whistleblower report. He appeared to be the first casualty of the expanding impeachment inquiry over the Ukraine call.

Volker was a key U.S. official involved in negotiations to help Ukraine stop the war in Donbas with Moscow-backed separatists.

Pompeo dodged questions about the phone call and the complaint during an interview with ABC’s “This Week” on Sept. 22, days before the White House released a summary of the call which showed Trump asking about the Bidens' dealings in Ukraine.

Pompeo had argued against releasing the transcript, saying it would set a bad precedent — never acknowledging he knew exactly what was in the call.

Asked about reports of the substance of the conversation, Pompeo said he wasn't familiar with them and couldn't comment on them. "You just gave me information about a [intelligence community] report, none of which we've seen," he said.

He then turned to the subject of former Vice President Biden, who's running for president.

"I do think if Vice President Biden behaved inappropriately if he was protecting his son and intervened with Ukrainian leadership in a way that was corrupt, I do think we need to get to the bottom of that," he said. "America can not have our elections interfered with, if that's what took place there."

The House Foreign Affairs Committee subpoenaed Pompeo last week, demanding that he turn over documents related to the phone call. (nbc/ez)